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Thalamic nuclei

Main nuclei of the thalamus.

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This is London. More specifically, it’s the air space above London which happens to be one of the busiest and most congested air spaces in the world. Each day over thirty five hundred flights either land or take off from one of the busy airports in this famous metropolis. Now, I know what you’re thinking. You think we provided you with some dodgy link to the wrong video and this is not actually the Kenhub video tutorial to learn about the thalamic nuclei. Well, believe it or not, you are in the right place and this is the correct tutorial.

Then why are we looking at planes? I hear you ask. Think it about this way. The thirty five hundred flights passing through London each day are monitored, controlled, and directed by the good people known as air traffic controllers. Yes, those unsung heroes of the sky, who hide away in their lofty control towers. These are the girls and guys who tell each pilot where to go, how to get there, when to land, when to take off, and even make sure they don’t land on the wrong runway.

So interestingly enough, your thalamus also acts like a kind of traffic controller in your brain. It tells your nervous impulses coming from all over your body including other parts of your brain where they need to go, when they are allowed to go, and where they can’t go. Sometimes, it tells them to wait or blocks them altogether in efforts to prevent the target destination becoming overloaded.

Just like any major control tower, there are several air traffic controllers doing this job. And guess what? Your thalamus is no different. But instead of air traffic controllers, it has individual groups of nerve cell bodies which are known as nuclei – each with a dedicated role for specific pathways within the brain. Without air traffic controllers, we would live in a world of continual travel mayhem, and similarly, without our thalamus, our brain would be a similar mess, with signals going to the wrong places, overloading our neural pathways, and causing complete overload of our mental capacity to do basically anything.

Are you intrigued to find out more? I hope so. Why not stay with me now as we explore the ins and outs of the thalamic nuclei.

So today we’re going to be talking about what the thalamus is and where about it’s located, the thalamic nuclei, and the basic functions of the thalamus. We’ll then look in more detail other parts of the thalamus, and we’ll finish by mentioning a clinical scenario where the thalamus is relevant.

So let’s start from the top and introduce the image we’ll be discussing today.

So in this image, we’re looking from the left side at the brain and the brainstem cutting the midsagittal plane with the thalamus highlighted in green here. This over here is the thalamus removed from the brain with the anterior here, the posterior here, the lateral sides here, and the midline here. And we sliced coronally through this left side of the thalamus and displaced the posterior part backwards so we can see what the thalamus looks like in cross-section. Each side of the thalamus is vaguely egg-shaped and is made of gray matter, and the two sides are connected by the interthalamic adhesion or connection.

The thalamus is part of the diencephalon, and this includes four parts – the thalamus along with the subthalamus, the epithalamus, and the hypothalamus. So, this image here nicely shows the thalamus in relation to its surrounding structures and we’re above the brain looking down at a large section of the cerebral hemispheres removed. And over here, we have the fornix wrapping around the thalamus to merge with the hippocampus.

So, the thalamus sits fairly centrally within the brain on either side of the midline, cuddled by the telencephalon and the ventricles. And its boundaries are anteriorly, we have the interventricular foramen and the head of the caudate nucleus; superiorly and posteriorly, we have the lateral ventricles over here and the fornices over here which hugs the thalamus; inferiorly, we have the tegmentum or the roof of the midbrain; medially, we have the lateral wall of the third ventricle; and laterally, we have the internal capsule and lateral ventricles.

So that’s all well and good, but why do we need this eggy thing in the middle of our brains?

The thalamus is a kind of sorting office for our senses. All sensory information is relayed through the thalamus in a thalamocortical direction – simply meaning that it goes from the thalamus and then it ends up in the cortex, and this includes information from the special senses of touch, temperature and pressure receptors and information from the cerebellum.

The sensory information is modulated by emotions and memory by the limbic system and by signals from the cortex. Inhibitory neurons synapsing in the thalamus are used to filter the sensory information reaching our conscious mind to ensure it’s not overwhelmed by sensory information. Motor information from the cortex is relayed and modulated in the opposite direction which is the corticothalamic direction and awareness, arousal, memory, and language also involve the thalamus. But to look at the functions properly, we need to identify the types of nuclei that are found within the thalamus, so we’re going to next talk about that.

So when talking about nerves, a nucleus is a collection of nerve cell bodies in the central nervous system. And there are several nuclei in the thalamus, one of which we’ve highlighted here in green, and we’ll get to this specific nuclei shortly, but just for now, all we need to know is that they can be split into three types – relay nuclei, association nuclei, and nonspecific nuclei.

So we’re going to begin, of course, with the relay nuclei, and as we can see, the relay nuclei such as the ventral lateral nucleus which we can see here modulate and relay sensory inputs to specific areas of the cerebral cortex – so from the body to specific area of the brain. Relay nuclei are also responsible for transmitting sensory signals to the cortex.

Association nuclei like the pulvinar nucleus highlighted here in green mostly receive inputs from the cerebral cortex, the limbic system, the basal ganglia, and the nonspecific thalamic nuclei. They then send signals back to the cerebral cortex, therefore, the direction is from one specific area of the brain to another specific area of the brain, and this facilitates communication between brain areas.

Nonspecific nuclei – for example, the highlighted thalamic reticular nuclei – receive input from the reticular formation or the arousal center in the brainstem as well as the cortex and other nuclei in the thalamus itself. The nonspecific nuclei project diffusely or to nonspecific targets to the cortex and the rest of the thalamus, therefore, from the brain and the brainstem to the thalamus and throughout the brain.

So, this explanation is a little bit complicated so just bear with me. So, nonspecific nuclei are thought to be responsible for arousal and attention and they do this by inhibiting certain pathways in the brain so that other pathways have priority. So, in other words, you could say that nonspecific nuclei help to block out what you could call electrical noise so that a particular pathway can pass through uninhibited. Failure of some of these nuclei can therefore cause effects such as hallucinations because of signals getting through that shouldn’t be getting through. That is, some of that electrical noise is interfering with the prioritized pathway.

Now that we’ve outlined the types of nuclei in the thalamus, let’s name some of the major nuclei and then look at each of them in turn.

The thalamic nuclei can be separated into five groups – anterior nuclei shown in purple, the medial nuclei colored in orange on the diagram, and the lateral nuclei which are the pink areas. These three groups of nuclei are separated by the medial medullary lamina highlighted in green. And within this lamina, there are the intralaminar nuclei highlighted now. And finally, the thalamic reticular covers the lateral aspect of each half of the thalamus like a shield as we can see.

So let’s begin talking about these nuclei and we’re going to start with most anterior nuclei, the anterior nuclei.

So the anterior nuclei are a group of three nuclei – anteroventral, anteromedial, and anterodorsal nuclei – with similar functions; so can be considered simply as one group – the anterior nuclei of the thalamus – and the group is highlighted on this image here in green. And as we can see, it sits in front of the medial medullary lamina and the anterior nuclei is closely associated with the limbic system, and this permits its functions in creating emotional states, attention, alertness, and memory acquisition.

The anterior nuclei receives afferent fibers from the mammillary nuclei of the hypothalamus, the dorsolateral prefrontal neocortex, and posterior neocortex. The mammillary nuclei are vital for memory functions while the neocortex is responsible for higher order thinking and planning. The group sends efferent fibers to the anterior limbic area, the cingulate gyrus, and the parahippocampal gyrus, which are all parts of the limbic system.

Okay, so crossing the medial medullary lamina in a posterior and medial direction, we now come to the medial thalamic nuclei, and this consists of the medial dorsal nuclei and the midline nuclear group. The medial dorsal nucleus highlighted here lies between the medial medullary lamina laterally and the midline nuclear group medially. And they are responsible for integrating sensory, motor, visceral and olfactory information and relating the signals to the individual’s emotional state, and it’s made up of two parts – the anteromedial magnocellular part and the posterolateral parvocellular part. These parts have an array of reciprocal connections, meaning, they receive afferent fibers from and send efferent fibers to the same area.

So let’s talk about each part.

The anteromedial magnocellular part has reciprocal connections with the ventromedial cingulate gyrus, the inferior parietal cortex, and the insular cortex. It also collects afferent nerves from the amygdala and the lateral nuclei of the thalamus, and efferent fibers from the posterior olfactory areas. From these connections, we can gather that the anteromedial magnocellular area of the medial thalamus is used in our perception of smell, emotional states, memory, our sense of self, and even homeostasis.

The posterolateral parvocellular part of the medial dorsal nucleus has almost exclusively reciprocal connections and these interactions are with the prefrontal cortex, the anterior cingulate gyrus, and the supplementary motor area. And these areas are involved in self-control, stimulus significance perception, outcome prediction, what we like to taste, emotional states, pain perception, and planning complex movements.

Okay, so now to talk about the other part of the medial nuclei – the midline nuclear group, also known as the periventricular nuclei. And this consists of three nuclei, but we’ll consider them as a group since they have similar connections and functions.

As with the other medial nuclei, this group has many reciprocal interactions and these are with the cingulate gyrus and the orbitofrontal cortex. The group’s merely afferent connections are from the hypothalamus, the periaqueductal gray matter of the midbrain, the spinothalamic tract, the reticular formation of the brainstem, and the nucleus accumbens. Its solely efferent associations are to the hippocampus and the amygdala. So this array of connections gives an indication that the midline nuclei are involved in memory, emotion, alertness, attention, and pain perception.

But enough about the middle of the thalamus, let’s visit the sides.

So the lateral thalamic nuclei are unsurprisingly found lateral to the medial medullary lamina, and they are the most divided group being split into two groups – the dorsal and ventral groups which are then further subdivided. And some of these divisions are then divided up again. Each area is a nucleus with specific connections and distinct functions. But first, we’re going to deal with the dorsal group.

So the lateral dorsal group of nuclei are made up of the lateral dorsal nucleus, the lateral posterior nucleus, and the pulvinar. And just remember that dorsal when referring to the brain means towards the top of the skull while ventral means down towards the body. And this orientation changes when we get into the brainstem and spinal cord, but we’ll not going to be looking at those today.

The lateral dorsal nucleus is the most anterior nucleus of the lateral dorsal group and it receives signals from the pretectum and the superior colliculus. It also has a number of connections with other areas which are unknown in nature and these areas are the cingulate gyrus, the parahippocampal cortex, the parietal cortex, and the presubiculum.

The pretectum has been implicated in optical reflexes such as pupil constriction, REM sleep, and the inhibition of pain pathways, and the superior colliculus is responsible for the ability to fix and follow moving objects. The thalamus likely integrates stimuli from these areas with information from other areas such as peripheral nerves, the cerebellum, and the cortex to permit or inhibit these reflexes and drives. Emotions, pain perception, alertness, memory and sensory information are also integrated with this information even if the nature of these connections is unknown.

The lateral posterior nucleus sits posterior to the lateral dorsal nucleus as seen here in the diagram, and this nucleus takes some signals from the superior colliculus, has reciprocal connections with the parietal lobes, and connections of an unknown nature with the cingulate gyrus and the parahippocampal cortex. This indicates functions similar to those of the lateral dorsal nucleus namely integration of emotions, pain, alertness, memory, and sensory information only without the optical connections and functions.

As now the final part of the lateral dorsal group, the pulvinar. The pulvinar is the big booty of the thalamus being the posterior most protrusion highlighted in green here, and it can be considered as a group of four distinct nuclei but they are very small print and have similar connection so we’re going to be considering the pulvinar as a whole. So its afferent connections include the superior colliculus, the retina and the association cortex, and it provides efferent innervation to the parietal, temporal and occipital cortices, the prefrontal cortex, and the cingulate gyrus.

The pulvinar also communicates reciprocally with other thalamic nuclei.

So, the exact functions of the pulvinar are not known, but given the array of connections, most notably the number of cortices receiving signals from the pulvinar, we can tell that its functions are pretty complex. So it’s involved in vision particularly visual attention as well as language and it’s thought to play roles in cognition, memory, sensation, and pain. Lesions of the pulvinar can result in hallucinations and language problems which is one of the main ways that we learn about the functions of an area.

Okay, so we’ve dealt with the lateral dorsal groups of the thalamus; now, it’s time to move on to the ventral lateral groups.

So, by convention, ventral comes before lateral while dorsal comes after lateral when naming the thalamic nuclei, and that’s just the way it’s done. So the ventral lateral group of nuclei are ventral inferior to the dorsal lateral group, anterior to the pulvinar, and lateral to the medial medullary lamina. So, who’s included in this exclusive group, I hear you ask?

Well, there are four nuclei and these are namely the ventral anterior nucleus, the ventral lateral nucleus, the ventral posteromedial nucleus, and the ventral posterolateral nucleus. So let’s, of course, begin anteriorly.

The ventral anterior nucleus is the anterior most nucleus of the ventrolateral group of nuclei and it receives afferent connections from the globus pallidus and substantia nigra to help make our voluntary movement smooth and the premotor cortex to help with visual-spatial coordination.

Reciprocal connections exist with the intralaminar nuclei and from the ventral anterior nucleus signals go to the frontal lobe and the anterior parietal cortex.

So, from these connections, we can appreciate that the ventral anterior nucleus has an important role in integrating signals for planning and initiating movements and it helps us to move smoothly and safely within our environment. So all those clumsy movements that you sometimes do might be down to your ventral anterior nucleus temporarily going rogue.

The efferent connections with the cortex indicate a role in sending information on movements and coordination to the conscious mind, but of course, to be truly coordinated, we need to synthesize information from the cerebellum as well bringing us nicely to the ventral lateral nucleus.

So sitting just caudal to the ventral anterior nucleus, the ventral lateral nucleus gets afferent signals from the cerebellum, the globus pallidus, the spinothalamic tract, vestibular balance nuclei, and the precentral motor cortex. And areas receiving efferent signals from the ventral lateral nucleus include the supplementary motor cortex, the lateral premotor cortex, and area four of the primary motor cortex.

So, the ventral lateral nucleus is used to relay proprioception and motor signals from the cerebellum and it’s firing all the time to relay information from passive and active movements. And moving caudally, we come to the ventral posterior nuclei, which consist of the ventral posteromedial nucleus and the ventral posterolateral nuclei, and they just sit anterior to the pulvinar.

So let’s just leave the medial medullary lamina for now and deal first with the ventral posteromedial nucleus. And this nucleus receives one main batch of signals and they come from the trigeminothalamic tract. This tract transmits pain, temperature, and crude touch from the areas innervated by the trigeminal nerve – that is the face and scalp as far back as the vertex of the skull which is where the frontal, parietal and occipital bones meet on the top of your head.

From the ventral posteromedial nucleus, the signals are then relayed to areas 1 and 3b of the primary somatosensory cortex, the secondary somatosensory cortex, and the insular cortex. And the ventral posteromedial nucleus therefore relays pain, temperature and crude touch sensations from the face and head to be registered in our consciousness.

So, what about the other ventral posterior nucleus?

So, the ventral posterolateral nucleus is kind of the headline act of the thalamus. It receives signals from the spinothalamic tract, responsible for carrying pain, temperature and the crude touch sensations from the body, and the dorsal medial lemniscal pathway, which transmits feelings of light touch, vibration and proprioception. And from the nucleus, signals go to the same places as those from the ventral posteromedial nucleus namely areas 1 and 3b of the primary somatosensory cortex, the secondary somatosensory cortex, and the insular cortex. And the ventral posterolateral nucleus relays pain, temperature and crude touch sensations from the body.

Both ventral posterior nuclei are arranged into curved lamellae and each lamella is responsible for a specific body area such as the hand. The neurons pass through the lamella in correspondence with the depth of the tissue they come from, the perception of which is called sensorial depth.

For example, a signal passing through the ventral most part of the lamella corresponding to the hand contains fibers from the superficial structures of the hand while the deeper structures pass through the anterodorsal region of the same lamella, and this allows us to distinguish between the so-called deep and superficial pains and sensations.

The efferent fibers for both the ventral posteromedial and posterolateral nuclei travel through the posterior limb of the internal capsule and through the corona radiata.

So next let’s look at the two nuclei which are a little bit more caudal and inferior. So, the geniculate nuclei, sometimes grouped together and called the metathalamus, are two nuclei – the medial geniculate nucleus and the lateral geniculate nucleus – and they are sometimes considered to be part of the lateral group of nuclei of the thalamus but are not included in the dorsal or ventral groups. And geniculate means knee-shaped, because they were thought to look vaguely like the knee in shape, only a little bit smaller and they protrude from the ventrolateral surface of the thalamus inferior to the pulvinar and each has distinct connections and functions, so first let’s talk about the medial geniculate nucleus.

So just about here hiding under the bulge of the pulvinar, the medial geniculate nucleus receives afferent connections from the superior colliculus, the inferior colliculus, and the brainstem auditory nuclei. And they sends efferents to the auditory cortex, the insular cortex, and the opercular cortex.

The function of the medial geniculate nucleus is to relay auditory signals to be consciously perceived to be integrated into memories and to modulate our alertness. Did they make you more alert? Signals through your medial geniculate nucleus definitely help with that.

The lateral geniculate nucleus has been very well described and it consists of six laminae which is simply numbered from ventral to dorsal. So laminae one, four and six receive signals from the contralateral nasal hemiretina, while laminae two, three and five receive information from the ipsilateral temporal hemiretina. There are also interlaminar cells, which are also called koniocellular cells which specifically transfer information from short-wavelength blue light cone cells.

The nucleus passes information onto efferents which go to area seventeen and extrastriate visual areas of the occipital cortex. So it’s clear that the lateral geniculate nucleus relays optical information from our retinas to the occipital cortex for processing and, more specifically, laminae one and two relays signals about movement, gross depth perception and brightness, and laminae three to six relay information on color, fine depth perception, texture and shape. So, in quick summary, the medial geniculate nucleus relays hearing while the lateral geniculate nucleus relays sight.

Okay so we’ve dealt with the main body of the thalamus now but there are two last sneaky groups of nuclei I still want to talk about, and they are the intralaminal nuclei and the reticular nuclei.

So let’s begin, of course, firstly, with the intralaminar nuclei which are held within the medial medullary lamina, and that’s highlighted just here. The medial medullary lamina is white matter arranged in a y-shape when viewed from above. And over here you can see the intralaminar nuclei and they’re included in the nonspecific ascending reticular activating system and that refers to its connections from the reticular nuclei of the brainstem and its diffuse innervation of the cerebrum and it’s thought to be responsible for controlling alertness and attention.

And let’s discuss the connections of each group starting at the front. So, primarily, the intralaminar nuclei are nonspecific nuclei and they can be split into two groups – anterior and posterior. So, our first one, anterior intralaminar nuclei, they receive afferents, the spinothalamic tract, the reticular nuclei of the brainstem, the superior colliculus, and the pretectal nuclei. And they communicate reciprocally and diffusely with the cortex. And they send efferents to the striatum.

The posterior intralaminar nuclei collects signals from afferents of the inferior pallidum, the pars reticularis of the substantia nigra, and pedunculopontine nuclei, and the spinothalamic tract, and they have reciprocal arrangements with the motor, premotor and supplementary motor areas. And their anterior counterparts also send efferent signals to the striatum.

The intralaminar nuclei as a unit transmit alertness and arousal stimuli and are vital in sensory-motor integration, and the posterior group of intralaminar nuclei are also involved in speaking and motivation.

Alright now, for the last but definitely not least of the thalamic nuclei – the reticular nuclei. So, the thalamic reticular nuclei are two curved lamellae of white matter which encapsulate the thalamus on its lateral aspects – one on each side – and they’re named reticular due to their fibrous network appearance as reticulum is Latin for little net. Each nucleus is stimulated from the thalamus by a thin layer known as the external medullary lamina positioned here and being made primarily of GABAergic neurons. The thalamic reticular nuclei are inhibitory in function.

The reticular nucleus does not communicate with the cortex but instead forms reciprocal interneuron connections with pathways running through the brain, and the most notable pathways are the thalamocortical pathway from the thalamus to the cortex, the corticothalamic pathway from the cortex to the thalamus, the thalamostriatal pathway from thalamus to striatum, and the pallidothalamic pathway from the pallidum to the thalamus.

And these connections allow the reticular nuclei to monitor and control activity of these pathways. Reticular nuclei neurons can be observed firing in response to sensory signals and, on top of this, they filter information being relayed to the thalamus by inhibiting extraneous signals.

Alright, so we finished discussing the nuclei of the thalamus. Great work!

So before we finish up, let’s have a bit of a chat about the thalamus and how the thalamus and its nuclei become relevant clinically. We’re going to be using the example of a thalamic stroke.

So, this diagram is showing the brain from the underside with a penetrating artery highlighted in green, and this is the anterior choroidal artery, and these penetrating arteries are where the problem arises in thalamic strokes. So, a stroke can either be ischemic – in other words, due to an inadequate blood supply to an area – or hemorrhagic – which is a bleed from one of the supplying arteries.

An ischemic stroke causes damage to an area because the area cannot sustain cell activity due to a lack of oxygen and nutrients and prolonged ischemia results in cell death and in the brain, this is the death of neurons. With hemorrhagic stroke, damage is done through the irritant effect of blood, local pressure and mass effect which refers to an expanding area – the bleed – within the skull vault causing compression of other regions of the brain. The pattern of dysfunction after a stroke correlates with the area affected.

Now given the functions of the several thalamic nuclei that we’ve already discussed, in the thalamus, a stroke affecting the ventral posteromedial and posterolateral nuclei completely blocks transmission of tactile sensory information or if it affects the medial nuclei, sensory-motor disturbance is observed. So, noting these disturbances and correlating with post mortem or CT findings is a very useful way of mapping the functions of specific thalamic areas.

So, the management of ischemic and hemorrhagic stroke are pretty different. So, for ischemic strokes, thrombolysis which is breakdown of the clot or fibrinolysis which is breakdown of connective fibers within the clot are often performed followed by long-term anticoagulant treatment whereas for hemorrhagic strokes, the main goal is really to stop the bleeding as stroke can cause death and disability, so early identification and suitable treatment is really the key to improving prognosis. So knowing the possible presentations of stroke including the varying presentations of thalamic stroke provides a greater chance at picking this up early and treating them successfully.

And we’re done. So before we finish, let’s just summarize what we’ve talked about today. So, first, we discussed the thalamus as a whole, its location atop the brainstem, its boundaries, its role in relaying, modulating and filtering sensory and motor signals, and also an awareness, alertness and memory and language.

We then named and described the location afferent and efferent and reciprocal connections as well as the functions of individual nuclei of the thalamus. So, we had the anterior nuclei, the middle nuclei which consists of the medial dorsal nucleic and the midline nuclear group, the lateral dorsal nuclei which are the lateral dorsal nucleus, the lateral posterior nucleus and the pulvinar, the lateral ventral nuclei which can be divided into the ventral anterior nucleus, the ventral lateral nucleus, the ventral posteromedial nucleus and the ventral posterolateral nucleus, the geniculate nuclei of which there is a medial and a lateral nucleus, the intralaminar nuclei which we can view as the anterior and posterior groups of nuclei, and the reticular nuclei. And to finish, we used the situation of a thalamic stroke to show why knowledge of the thalamic nuclei is important clinically.

Okay, so thanks for watching this Kenhub tutorial, and have a great day. Happy studying!

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